1. Field of the Invention
The present invention relates to a longitudinally coupled resonator type surface acoustic wave filter, a surface acoustic wave device including such a filter, and a communication device including such a filter or such a surface acoustic wave device.
2. Description of the Related Art
In recent years, the number of subscribers to communication devices such as portable telephones and other devices has increased, and the services of the communication devices have become more diversified. For this reason, communication devices of which the transmission-side frequency band and the reception-side frequency band are located near each other have been increased in number. Moreover, it is necessary to provide a desired amount of attenuation in the immediate vicinity of a pass band, depending on the types of communication devices so that interference between the communication devices and other communication devices can be prevented. For surface acoustic wave filters widely used as band-pass filters in the RF stages of portable telephones, it has been more intensively required to provide a desired amount of attenuation in the vicinity of a pass band.
Moreover, for the purpose of reducing the number of parts contained in such a surface acoustic wave filter, it has been increasingly demanded to provide a balanced-unbalanced signal conversion function, a so-called balun function, for the surface acoustic wave filter. For this reason, the use of longitudinally coupled resonator type surface acoustic wave filters as band pass filters in the RF stages of portable telephones has become the standard, since the filters can easily provide the balanced-unbalanced signal conversion function. For example, Japanese Unexamined Patent Application Publication No. 5-267990 discloses such a longitudinally coupled resonator type surface acoustic wave filter.
In the above-described longitudinally coupled resonator type surface acoustic wave filters of the related art, disadvantageously, deterioration of a “shoulder”-characteristic (the reduction in steepness of a frequency characteristic), called transversal response is caused on the higher frequency side of a pass band. Therefore, there are problems in that attenuation on the higher frequency side of the pass band is not satisfactory, although it is required by communication devices which operate in a PCS system (Personal Communication System) or other telecommunications systems. As a filter having a large amount of attenuation on the higher frequency side of a pass band, for example, Japanese Unexamined Patent Application Publication No. 10-126212 discloses a ladder type surface acoustic wave device. However, it is problematic that the balanced-unbalanced conversion function can not be provided by ladder type surface acoustic wave filters. That is, according to the related art, it has been difficult to produce a surface acoustic wave device having a sufficient amount of attenuation on the higher frequency side of the pass band and also having a balanced-unbalanced conversion function.